1. Field of the Invention
This invention relates to cross-connection field arrangements, and particularly to switching system main distributing frame network arrangements.
2. Description of the Prior Art
The adoption of the principle of centralization of common switching equipment produced the need to provide flexibility of interconnection between outside cable plant and central office equipment. Thus, a main distributing frame evolved which provides terminations for outside plant cable pairs on a fixed basis on one side of the frame with similar terminations for inside plant conductors on the other side of the frame. Wire cross connections or jumpers, manually placed, then connect the desired central office circuit to the proper outside plant cable pair. Accordingly, the main distributing frame, in essence, constitutes a cross-connection field which increases in size in direct relation to office size. Although substantial technological improvements have occurred in automatic switching systems in intervening years, the basic main distributing frame design presently in use throughout the world has not changed in over half a century.
The retention of the original main distributing frame arrangement is attributable, in part, to certain advantages which it presents. For example, it is completely nonblocking in nature. Thus, any particular outside plant cable pair is capable of physical connection to an appropriate central office circuit by placing a cross-connection pair. The physical termination of conductors also provides a point for maintenance test access. In addition, the physical equipment of which the main distributing frame is constructed consists principally of iron work and terminal strips, thus constituting a relatively low plant investment. However, these factors are no longer sufficiently advantageous when it is recognized that, although the plant investment is low, the continuing need for investment in labor is extremely high. In addition, existing main distributing frame arrangements in many cases have grown far beyond their initially estimated sizes, thus forcing unreasonable measures to be taken to provide the necessary capacity. Massive reterminations, the physical requirement that portions of the distributing frame be segregated and interconnected with large cross-connection tie cables, and the phenomenal growth of individual switching system offices have contributed to maintenance problems, which in some instances have rendered many cross-connection changes impossible or at least prohibitively expensive. And, finally, cross-connection jumper congestion, massive record-keeping problems and the limitation of access for test purposes further compounds the disadvantages in view of the physical complexity of present day distributing frames.
Approaches to solving the main distributing frame problems have included the development of modular manular frames and the development of automated frame administrative systems such as a system for preferential assignment of jumpers for use with either the existing frame or the modular frame. However, it has been found that such improved manual systems are not appropriate for many applications such as large metropolitan wire centers and centers containing large numbers of pairs such as trunks and special service circuits not subject to preferential assignment and other automated frame administration systems.
Conventional call-switching technology could be readily applied to the main distributing frame. However, because of the large switching capacity which would be required, a call-switching distributing frame would be prohibitively expensive. Such an application of call-switching technology fails to take advantage of the relatively low switching speeds and low frequency of operations (e.g., about one change per circuit per year on the average) which are required in the distributing frame as compared with the relatively fast speeds and response required in conventional switching machine applications.
U.S. Pat. No. 3,562,435 issued Feb. 9, 1971 to A. E. Joel, Jr. teaches a switching system arrangement in which the main distributing frame function is automated through implementation of an automatically controlled rearrangeable switching network. Although the Joel teaching constitutes a substantial advance in the art, it still has certain disadvantages including the use of "active" switch devices such as relays and contacts which would add significantly to the cost of the distributing frame.
U.S. Pat. No. 3,763,325 issued Oct. 2, 1973 to J. G. Kappel et al. discloses a distributing frame manual switch network which also represents a significant advance. However, Kappel et al.'s system also has certain disadvantages including the manual placing of jumpers or connections in the network stages, the lack of full flexibility in allowing connections between terminals on the same side of the network, and the inability to configure installations in a broad range of sizes.
Accordingly, it is an object of this invention to provide a distributing frame system which permits the automatic establishment of cross connections and connections for test access without a prohibitive increase in plant investment.
Another object is to provide a main distributing frame system permitting complete flexibility of interconnection and growth of the system from an initially installed size to any of a broad range of ultimate sizes.